The present embodiments relate to contactless transmission of at least one differential signal between a transmitter and a receiver given the existence of at least one common-mode noise signal. In particular, the present embodiments may relate to contactless transmission of at least one common-mode noise signal, which has a low frequency in comparison with at least one signal to be transmitted.
Data or signals may be transmitted in contactless fashion between modules. For example, in medical technology, a computer tomography has a rotating part and a stationary part, between which data is transmitted.
The contactless wideband transmission of differential high-frequency signals between a transmitter and a receiver, such as the two different modules, may have relatively strong and low-frequency common-mode interference effects, which impair the quality of transmission. The signal to be transmitted exhibits a spectrum in which the signal power in the range of high frequencies is distributed over a wide frequency band. Whereas the signal power in the frequency range beneath a limit frequency is insignificantly low. The lower limit frequency is in turn extremely low in comparison with the bandwidth of the signal. Typically, the bandwidth should be at least ten times as great as the lower limit frequency.
A differential driver serves as a rule to feed the signal into an antenna with two inputs with their polarity inverted with respect to one another, which generates an electromagnetic field that can be described as an overlay of two fields. The one field is characterized by feeding the same signal into both inputs (common mode) and the other field is characterized by signals inverted with respect to one another at the inputs (push pull). A metallic coupling structure, which is not in contact with the antenna but is situated in the immediate vicinity of the antenna, is used for contactless tapping of the field of the antenna in the close-up range. At the two outputs of the metallic coupling structure, two signals are tapped. The two signals can be split into a skew-symmetric portion and a straight-symmetric portion. A signal fed equally into both inputs of the antenna does not produce a skew-symmetric signal portion in the coupling structure output. An inverted signal should not produce any straight-symmetric signal portions.
The two weak signals at the coupling structure outputs are forwarded by way of two lines to a differential amplifier in the receiver. The differential amplifier should in an ideal situation amplify the skew-symmetric signal portion, which is available for further signal processing. The two signals, at the input of the differential amplifier, do not exceed the permissible working range of the amplifier input. If the working range of the amplifier input is exceeded, then errors occur with a frequency, which is no longer tolerable in the case of a data signal to be transmitted with regard to the subsequent clock or data recovery.
The danger that the permissible working range of the amplifier input will be exceeded exists if the straight-symmetric signal portion, such as the common-mode signal becomes too great at the input of the differential amplifier. Ideally, the common-mode signal would not be present.
There are three reasons for the occurrence of the common-mode signal. A first cause may be imbalances in the output stages of the line drivers can cause common-mode interference effects. A second cause may be an imbalance in the coupling, for example, as a result of positioning errors or manufacturing tolerances, can bring about a common-mode signal correlated with a useful signal.
A third cause for the occurrence of common-mode interference (so-called common-mode signal) is a mismatch of potential between the ground reference systems of the transmitter and receiver in the case of unequal reference systems. If the field caused by the mismatch of potential of the ground reference systems in the area of a coupling structure has the property such that the difference in potential of both ground potentials couples into both coupler halves with equal strength, then the interference is present as a common-mode signal at the input of the differential amplifier in the receiver and overlays the differential useful signal there. If a tolerable level is exceeded, an error-free clock and data recovery or a clock and data recovery, which is errored up to a tolerable extent, is no longer possible.
If the field caused by the mismatch of potential of the ground reference systems couples into both coupler halves with a different strength, then one part of the voltage is fed between the two reference potentials as a push-pull signal into the two lines to the amplifier input and overlays the differential useful signal. A suppression by the differential amplifier is not possible. A satisfactory clock and data recovery can also be endangered as a result.
The first-mentioned portions of the common-mode interference caused by imbalances in the amplitude are considerably smaller than the amplitude of the differential useful signal, with the result that they normally do not lead to overdriving of a working range of the amplifier input and can be tolerated. The common-mode interference effects caused by the voltage between the two ground potentials can exceed the useful signal amplitude by orders of magnitude, such that the signal quality drops intolerably as a result of the then unavoidable overdriving of the amplifier input.